Conditions have been found to allow the successful preparation of BR- liposomes by the reversed phase evaporation technique using the native purple membrane rather than free BR-monomers (prepared by pretreatment with detergent) that was previously used. Significantly higher incorporation levels of proton-pumping BR have been obtained with this technique. The extent of H+-pumping is markedly increased by raising the internal Pi from 50 to 200 mM. A major experimental problem is the ability to measure weak fluorescent signals, specific for deltapH and delta/psi probes, in the presence of extremely high levels of white actinic light. A combined fiber optics, electronic system was built to solve this problem. Further development of this system is underway to improve the signal to noise ratio.